Editing Coal (section)

==Composition==
Coal is a mixture of diverse organic compounds and polymers. Several kinds exist, with variable dark colors and composition. Young coals (brown coal, lignite) are not completely black. The two main black coals are bituminous, which is more abundant, and anthracite. The type of coal with the highest percentage of carbon in its chemical composition is anthracite, followed by bituminous, then lignite, and finally brown coal. The fuel value of coal varies in the same order. Some anthracite deposits contain pure carbon in the form of [[graphite]].

For bituminous coal, the elemental composition on a dry, ash-free basis of 84.4% carbon, 5.4% hydrogen, 6.7% oxygen, 1.7% nitrogen, and 1.8% sulfur, on a weight basis.<ref name=Perry/> This composition reflects partly the composition of the precursor plants. The second main fraction of coal is ash, an undesirable, noncombustable mixture of inorganic minerals. The composition of ash is often discussed in terms of oxides obtained after combustion in air:
{| class=wikitable
|+ Ash composition, weight percent
| {{chem2|SiO2}}|| 20–40
|-
| {{chem2|Al2O3}}||10–35
|-
| {{chem2|Fe2O3}}||5–35
|-
| CaO||1–20
|-
| MgO||0.3–4
|-
| {{chem2|TiO2}}||0.5–2.5
|-
| {{chem2|Na2O}} & {{chem2|K2O}}||1–4
|-
| {{chem2|SO3}}||0.1–12<ref>Combines with other oxides to make sulfates.</ref>
|-
|}

Of particular interest is the sulfur content of coal, which can vary from less than 1% to as much as 4%. Most of the sulfur and most of the nitrogen is incorporated into the organic fraction in the form of [[organosulfur compound]]s and [[heterocycle|organonitrogen compound]]s. This sulfur and nitrogen are [[covalent bond|strongly bound]] within the hydrocarbon matrix. These elements are released as SO<sub>2</sub> and NO<sub>x</sub> upon combustion. They cannot be removed, economically at least, otherwise. Some coals contain inorganic sulfur, mainly in the form of [[iron pyrite]] (FeS<sub>2</sub>). Being a dense mineral, it can be removed from coal by mechanical means, e.g. by [[froth flotation]]. Some sulfate occurs in coal, especially weathered samples. It is not volatilized and can be removed by washing.<ref name= KO>{{cite book |doi=10.1002/0471238961.0315011222151818.a01.pub3 |chapter=Coal |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2016 |last1=Hower |first1=James |pages=1–63 |isbn=978-0-471-48494-3 }}</ref>

Minor components include:
{| class=wikitable
|+ Average content
! Substance !! Content
|-
| [[Mercury (element)|Mercury]] (Hg)
|align=right| {{val|0.10|0.01|u=[[Parts-per notation|ppm]]}}<ref name="lab_hg">{{cite web|title=Mercury in coal: a review; Part 1. Geochemistry |author=Ya. E. Yudovich, M.P. Ketris |url=http://www.labtechgroup.com/support/hg-1-yudovich.pdf |archive-url=https://web.archive.org/web/20140901135845/http://www.labtechgroup.com/support/Hg-1-Yudovich.pdf |archive-date=1 September 2014 |date=21 April 2010 |access-date=22 February 2013 |publisher=labtechgroup.com |url-status=dead}}</ref>
|-
| [[Arsenic]] (As)
|align=right| {{nowrap|1.4–71 [[Parts-per notation|ppm]]}}<ref name="usgs_fs2005">{{cite web|title=Arsenic in Coal|url=http://pubs.usgs.gov/fs/2005/3152/fs2005-3152.pdf|date=28 March 2006|access-date=22 February 2013 |publisher=pubs.usgs.gov|url-status=live|archive-date=9 May 2013|archive-url=https://web.archive.org/web/20130509051812/http://pubs.usgs.gov/fs/2005/3152/fs2005-3152.pdf}}</ref>
|-
| [[Selenium]] (Se)
|align=right| {{nowrap|3 [[Parts-per notation|ppm]]}}<ref name="acs_env">{{cite book|title=Selenium in Our Environment – Trace Elements in the Environment|volume=123|page=96|doi=10.1021/ba-1973-0123.ch006|date=1973|chapter=Selenium in Our {{sic|Enviro|ment|nolink-y}} |series=Advances in Chemistry|last1=Lakin|first1=Hubert W.|isbn=978-0-8412-0185-9}}</ref>
|}
As minerals, Hg, As, and Se are not problematic to the environment, especially since they are only trace components. They become however mobile (volatile or water-soluble) when these minerals are combusted.